The correction of misalignments in teeth is achieved by the aid of brackets. The brackets are adhered onto the teeth of the respective jaw of the patient in predetermined positions and are subsequently joined with one another by a resilient arch wire. The brackets are applied to the individual teeth in a position that is selected so that at the end of the orthodontic treatment the arch wire extending through the brackets is substantially free from undulations. At the beginning of the treatment, the arch wire therefore has a very complex extension. The relatively large arch wire following the jaw is, depending on the misalignment of the teeth, more or less undulated within and outside the plane defined by the arch wire.
Two fundamental methods exist to determine the correct position of the brackets on the teeth to which they are to be adhered. One of these methods requires an optical measuring of the jaw. The measured data collected is then subsequently processed by the aid of a computer program which does not only determine the bracket position but also controls a wire bending machine. By means of the wire bending machine, a set of arch wires of different cross sections is bent to relatively complex shapes which take the individual misalignment of the teeth into consideration. This technique is particularly useful where lingual brackets are desired since the shapes of teeth on their lingual side may differ from individual to individual, making orientation difficult with respect to the contour of the tooth crown. This is different from the image of the crown on the labial side where teeth have essentially matching shapes despite the individuality of human beings.
The above-mentioned computer technology is extremely expensive. It requires an investment on the scale of EUR 100,000 to EUR 120,000 for machines and programs. In view of the relatively small number of orthodontic treatments using lingual brackets, the acquisition of such equipment is inefficient for the individual orthodontist. As a matter of fact, less than 10 computers of this type are being used worldwide at present.
The conventional method of defining bracket positions on teeth is the so-called Hiro technology. In accordance with this technology, the orthodontist first makes a dental impression of the denture to be corrected. From this impression the dental technician prepares a plaster cast of the denture to be corrected. The cast taken from the impression is sawed into individual teeth that are subsequently reassembled by means of wax in a manner replicating the perfect occlusion sought to be achieved by the orthodontic treatment.
Afterwards, the arch wire is made. Brackets are attached onto the arch wire and the wire is positioned together with the brackets on the lingual side of the corrected plaster cast in a way where the brackets oppose the teeth in a predetermined manner. A predetermined distance is kept between the base plates of the brackets and the teeth since this shall later be filled by a casting compound.
Individual small caps are formed from a light-curing acrylic resin over the incisal edge of each tooth, these caps extending to the brackets and adhering thereto. After curing of the small caps, the brackets are removed from the arch wire. Then, the small caps with the brackets hanging thereon are individually set onto the teeth of the corrected plaster cast. The gaps between the base plates of the brackets and the tooth crowns are filled by a curing casting compound, forming a so-called individual base for each bracket. It is self-evident that a separating agent is used at the teeth of the corrected cast to prevent adhesion of the casting compound to the teeth.
After completion of this preparatory laboratory work, the set of brackets is adhered by the orthodontist using a thin adhesive film onto the corresponding teeth of the denture to be corrected on the patient. Here the small caps again serve as gauges to position the brackets on the teeth in the same manner as they had been positioned on the corrected plaster cast. After curing of the adhesive, the small caps are then broken off. This is an elaborate job, since the acrylic material must be removed completely from the brackets.
A problem with this procedure is that, after an unintended release of a bracket from a tooth sometime during the orthodontic correction treatment, it is difficult to correctly re-attach the respective bracket onto the tooth. This work can only be carried out with the assistance of a laboratory. When using a computerized measurement, a completely new measurement is required. A new set of wires must be made, since the patient usually does not immediately consult his/her orthodontist and the alignment correction may have, in the meantime, reset itself. With the Hiro technology, a new individual base including an acrylic cap must be manufactured in the laboratory for the new bracket.
The unintended release of a bracket is not rare. It usually occurs at least once with every patient during the duration of the treatment.